This invention relates to a RM.sub.5 -type (e.g., SmCO.sub.5) rare earth permanent magnet, a method of heat treatment of the same, and a magnetic body having a specific outer shape, and more particularly to a rare earth permanent magnet of a large size required for a corpuscular ray accelerator and an image diagnostic device.
SmCo.sub.5 -type rare earth permanent magnets have heretofore been used as small size high-performance permanent magnets. For producing a SmCo.sub.5 -type permanent magnet, an alloy consisting of 65.75 to 66.0 wt. % Co and the balance Sm is first melted by high-frequency melting in an Ar atmosphere, and the molten alloy is cast into an ingot, and the ingot is pulverized in a protective atmosphere into fine powder by a ball mill or the like. The thus obtained powder whose particle size is several .mu.m is compressed and compacted by a mold (which is disposed in a magnetic field) into a compact, and this compact is sintered at a temperature of not less than 1100.degree. C. Then, the resultant sintered product is again maintained at 950.degree. to 1000.degree. C. for 1 to 2 hours in an Ar atmosphere, and then is cooled in a furnace at a rare of 0.1.degree. to 3.degree. C./min. After the sintered product is cooled to a temperature of 770.degree. to 830.degree. C., the sintered product is quenched in oil or in a sand-Ar fluid bed. Such a heat treatment method is disclosed in Solid Communications, 8 pp. 139 to 141 (1970). In the heat treatment, it is necessary that the sintered product should be cooled in the furnace to a low temperature between the sintering temperature and a temperature lower than the sintering temperature by about 300.degree. C. (usually, the sintered product having been maintained for a predetermined period of time is cooled in a furnace to a low temperature lower than the sintering temperature by 500.degree. C. or less), and then is quenched to a temperature not more than about 300.degree. C. When the above quenching treatment is not applied to the SmCo.sub.5 -type rare earth permanent magnet, the coercive force iHc is greatly lowered due to so-called Westendorph effect (i.e., a phenomenon in which the coercive force iHc exhibits an extremely small value at a specific temperature) as clearly described in the above-mentioned technical report, and the resultant permanent magnet fails to have a high coercive force which is a feature of the SmCo.sub. 5 -type permanent magnet, and therefore can not be suited for practical use. Therefore, in the heat treatment of SmCo.sub.5 -type magnets, the decrease of the coercive force iHc due to the Westendorph effect has been avoided by the quenching in oil, the quenching by the fluid bed or the quenching by gas-jetting (or a water-cooling quenching for a very small-size magnet), thereby providing the permanent magnet of a high coercive force. There is also known a SmCo.sub.5 -type rare earth permanent magnet of a higher performance which is composed of composite components. Such magnet consists, by weight, of a rare earth metal (23 to 30% Y, 32 to 40% Ce, 34 to 42% Sm, or 32 to 40% Pr) or 34 to 42% of a mixture (mesh metal) thereof, and the balance Co (see Japanese Patent Examined Publication No. 48-364).
Because of its high magnetic characteristics, the magnetic flux amount of the above SmCo.sub.5 -type anisotropic rare earth permanent magnet is large per unit volume of the permanent magnet. Therefore, this permanent magnet, when used in conventional audio parts, automotive electric parts, and computer and office automation parts, has been designed to have as small a size as possible. Recently, however, there has been an increasing demand for a large-size rare earth magnet for use in a part of a corpuscular ray accelerator, such as a wiggler, an undulator and a high vacuum pump, a drive source for a servo motor or the like, and an image diagnostic device.
Particularly, the SmCo.sub.5 -type permanent magnet has a high coercive force and a high Curie point of 710.degree. C. and is excellent in heat resistance and corrosion resistance, and therefore there has been a demand for a large-size, integral SmCo.sub.5 -type permanent magnet in the fields of the automotive and aircraft electric parts and the accelerator-related part which particularly require an excellent thermal stability.
When quenching such a large-size permanent magnet, there is encountered a problem that cracks develop in the permanent magnet. For example, with respect to the permanent magnet for a wiggler, even a small-size magnet weighs 200 to 500 g per block, and a large-size magnet weighs more than 2 kg per block. In the quenching of such a large-size permanent magnet, in addition to the frequent development of cracks and fracture, the cooling effect does not proceeds into the interior of the magnet because of its large volume, and besides desired magnetic characteristics can not be obtained. To prevent such cracking and fracture, it has been proposed to employ a heat treatment of a recuperative oil-cooling system used for the quenching and tempering of steel; however, with this heat treatment, desired magnetic characteristics can not still be obtained. The reason for this will be mentioned. The SmCo.sub.5 -type permanent magnet has a thermal expansion coefficient of 6.6.times.10.sup.-6 /.degree.C. in the direction of the C-axis of the crystal grain (which constitutes the permanent magnet) and a thermal expansion coefficient of 12.6.times.10.sup.-6 /.degree.C. in the direction perpendicular to the C-axis. Therefore, when a large temperature difference occurs between the interior and surface of the permanent magnet at the time of the quenching, a tensile stress is induced on the surface of the magnet which cools faster.
For the above reason, the large-size anisotropic rare earth permanent magnet must be assembled by bonding a plurality of block-like permanent magnets together by an adhesive. However, the adhesive exists in the boundary
t magnets to form magnetic gaps, and the magnetic flux density is greatly decreased at these magnetic gaps, which results in a problem that the uniformity of the overall magnetic characteristics is adversely affected, thus adversely affecting the overall performance of the device. Further, the above-mentioned wiggler is used under high vacuum and in an environment in which radiation including ultraviolet rays is present. Therefore, there is also encountered a problem that the adhesive performance is deteriorated due to the evaporation of the adhesive under high vacuum and the application of the radiation to the adhesive. A further problem is that the above assembling by the bonding using the adhesive is an extremely complicated operation, and therefore requires much time and labor, and also makes it difficult to provide the product of a uniform quality.